Plastic deformation and alloying

Question 1

State three categories of material defects.

Answer 1

1. Point defects -
* Vacancies
* Interstitial atoms
* Substitutional atoms

2. Line defects -
* Dislocations

3. Volume defects -
* Grain Boundaries - interface between two crystal structures
* Particle Inclusions - differing crystal structure “internal” grain boundary
* Cracks and voids - may arise from processing route or during service

Question 2

Simply describe the different types of Point defects.

Answer 2

• Vacancies: An atom is missing in the lattice
• Self-intersitial: An atom in the wrong place

When one type of atom is directly dissolved into another solid state we get:
* Interstitial atoms: impurity atoms in interstitial sites
* Substitutional atoms: impurity atoms which substitute into the lattice.
* Note: these final two is how alloying works. By definition an alloying atom is a point defect.

Question 3

Simply describe the different types of Line defects.

Answer 3

These line defects are called dislocations and have two types:
* Edge dislocation: This is where there is an extra half plane of atoms in the lattice. The Burgers vector ,b, denotes the magnitude and direction of the lattice distortion associated with a dislocation, its perpendicular to the dislocation line.

• Screw dislocation: This is a spiral defect where a shearing force is applied and the lattice shifts in or out of the orginal lattices plane (i.e. in and out of a plane of paper if square is drawn on paper). Therefore the Burgers vector is parallel to the dislocation line.

Question 4

What causes plastic deformation?

Answer 4

Dislocations (Dn) assist/enable plastic deformation. ‘Slip’ causes dislocation motion.

Question 5

What is meant by ‘Slip’?

Answer 5

‘Slip’ is when dislocations moves, which allows plastic deformation. ‘Slip’ requires the breaking and reforming of one row of atoms at a time - much lesss energy than slipping a perfect plane.

Question 6

How do crystaline structures effect dislocation motion?

Answer 6

Dislocation motion occurs easily on close-packed planes and directions. There are slip systems in each crystal structure which affects how easliy dislocation motion occurs. More slip systems means easier to occur. Thats why FCC has some of the highest number of slip systems 12.

The number of slip systems thus effect the mechanical properties of different materials.

Question 7

What is mechanical twinning?

Answer 7

Mechanical twinning is an alternative deformation via homogenous shear of plane of atoms. i.e. the whole crystal plane will shear at once rather than along individual individual slip planes that are often smaller spacing.

Stress for twinning is often greater and less temperature dependent than stress for slip.

Twinning often occurs in BCC and HCP metals at low temperatures, but rarely in FCC metals.

Question 8

What are the effects of temperature on Yeild stress for different Crystaline structures?

Answer 8

• FCC: Temperature independent since slip is easy
• BCC: Temperature dependent since no close packed planes
• HCP: Temperature dependent since limited cloise-packed slip systems

Note:
* Slip is difficult at low temperatures and requires thermal activiation (for example in some of the BCC slip sytems). There is an element of diffusion in atom movement.

Question 9

What is the associated stress field for dislocations?

Answer 9

There is a compressive stress at the top half of the line dislocation (as there is too many atoms near each other). Then there is a tensile stress at the bottom half (because there is a gap bigger than usual).

Question 10

What happends at the yield point / yield stress, regarding discloactions?

Answer 10

Discolation motion initiates

Question 11

How do you increase yiled strength?

Answer 11

preventing dislocations from moving. For example by alloying

Question 12

What four key factors effect alloying?

Answer 12

1. Atomic size factor: <15% difference for substitiutional
2. Crystal structure: both structures (often) have to be the same for good solubility.
3. Electronegativity: if widely different (in periodic table groups), reaction may occur instead of solidifying.
4. Valence: metals more ealisy disolve in others of higher valency.

Question 13

How does alloying cause solid solution strengthening which increases yeild strength?

Answer 13

• Alloying may form a solid solution: solute atoms may take up interstitial or substitutional sites:
• There are stress fields associated with dislocations, and alloying effects them.
• When an solute atom is smaller than the solvent atoms, this will lead to a tensile stress on the surrounding atoms as they try to remain at their fixed distances.
• LIkewise for a solute atom bigger than the solvent atoms, this will provide a compressive stress on the surrounding atoms as they are pushed away fromt their desired spacing and want to return.
• These two propertites above mean that disolcations stress fields can be reduced in the tensile or compressive regions, so that when they move they may come in contact with other stresses from the solute atoms in the lattices which is higher stress. Thus needing more energy to allow the dislocation to move past.

Question 14

Why is steel’s Stress-strain graph like this, using alloying understanding and dislocation motion?

Answer 14

The carbon atoms aren’t usually able to fit in the lattice structure. However, they can fit in the tensile regions of a dislocation. This pins the dislocation, requiring more stress to move the disloacation.